The present invention relates to a method of determining the isomer composition of an isocyanate isomer mixture during isocyanate production processes, and to a method for the regulation or control of an isocyanate isomer production system for mixing or separating isomers. This invention also relates to an apparatus for the production of an isomer mixture with a setpoint isomer composition.
From the prior art, it is known to produce an isomer mixture of isocyanates, with a particular isomer composition, by means of an isomer production system.
For example, isomer separation can be carried out by means of distillation or crystallisation. Alternatively, a particular isomer mixture may be produced by mixing suitable initial isomer mixtures. The quality of the isomer mixture, for example in distillative isomer separation, can be regulated roughly through the process parameters of pressure and temperature, as well as the distillate/bottom-product ratios and reflux ratios. A disadvantage of this, however, lies in that with high product purities, the pressure and temperature provide almost no useful information about the concentration of the isomers. In other words, the sensitivity of the concentration determination is on the order of the measurement noise since the boiling points are close together. Furthermore, no physical method is yet known to be suitable for the determination of isocyanate isomer mixtures.
Previously, therefore, this quality monitoring has been carried out by taking samples and by, for example, subsequent manual chromatographic analysis, preferably gas chromatography (GC), of these samples. In the case of isocyanates, it is necessary to take occupational safety and environmental protection conditions into account, in order to avoid risks involved in the handling of such chemical substances. Furthermore, the number of samples that can be realistically taken is limited due to the associated labor cost, and information about the composition of the sample is not available until after a significant delay. Thus, in order to control the product quality of crystallizers or distillation columns, this manual method has significant disadvantages. This is particularly a problem since it does not allow any persistent trend to be established with respect to the concentration changes in many equipment components of a complex system.
With manual controls and sampling, it is conceivable that the isomer mixture being produced may have a relatively large difference in isomer content from the setpoint composition, particularly over relatively long periods of time. This can result in a reduction of the product quality or the production of waste.
Process chromatography or automated titration are relevant online methods for analyzing or assessing the isomer composition of an isomer mixture. A feature common to these methods is that the result is only available with a significant time delay after lengthy measurement times. Furthermore, these methods are characterised by elaborate sample delivery means, susceptibility to interference, and sizable consumption of the auxiliary agents and other consumable materials.
Monitoring and regulation of the isomer composition is important, and particularly, for the production of isocyanates. In this context, various isocyanates A, B, C, D, etc. consist of a mixture of two or more isomers 1, 2, 3, . . . , n.
These isocyanates include, for example, be naphthylene diisocyanate (bis-[isocyanate]naphthylene), xylylene diisocyanate (bis-[isocyanatomethyl]benzene), methylene diphenyl diisocyanate (MDI) or toluene diisocyanate (TDI), as well as other aromatic, alicyclic or aliphatic isocyanates, and mixtures thereof. In general, isocyanate intermediate or commercial products consist of the various isomers in different ratios.
Industrially, such isocyanate intermediate or commercial products are produced from an initial isocyanate mixture (i.e. a raw mixture) of a plurality of isomers 1, 2, 3, . . . , n.
For example, isocyanate A may be toluene diisocyanate (TDI), an isomeric mixture of the isomers 2,4-TDI (2,4-bis-[isocyanate]benzene), 2,6-TDI (2,6-bis-[isocyanate]benzene), 2,3-TDI and 3,4-TDI. The initial mixture may be separated into its isomers in order to achieve special high-quality product properties. For instance, the initial mixture may be separated into a commercial product I with 100% 2,4-TDI, and/or a commercial product II with about 65% 2,4-TDI and about 35% 2,6-TDI. Commericaly products such as these are available on the market. Another example is isocyanate B, which may be methylene diphenyl diisocyanate (MDI), an isomeric mixture of the isomers 2,2′-MDI (bis-[2-isocyanato-phenyl]methane), 2,4′-MDI (2-isocyanatophenyl)-(4-isocyanatophenyl)methane, 4,4′-MDI (bis-[4-isocyanatophenyl]methane) and other isomers with a higher ring number (i.e. more than 2 ring compounds).
The initial isocyanate mixture may be separated into its isomers in order to achieve special high-quality product properties. For instance, a commercial product I with 100% 4,4′-MDI and a commercial product II with about 50% 2,4′-MDI and about 50% 4,4′-MDI, both of which are available on the market.
It is absolutely necessary that the monitoring of the isomer composition be maximally accurate for compliance with a predetermined product specification. This monitoring must provide the composition as quickly as possible, so that the isomer system can be adjusted efficiently. Fast and maximally precise monitoring is particularly important since, due to the production technique, coupled products may be generated in the isocyanate isomer production.
The previously employed methods can meet these requirements only with significant limitations. In the offline gas-chromatographic examination, for example, the samples have to be taken and transported to the laboratory, where the sample is then prepared and subsequently analysed by gas chromatography.
An alternative to gas chromatography and titration, for quantitative analysis of the composition of substance mixtures, are the known spectroscopic methods from the prior art. These include, for example, near-infrared (NIR) spectroscopy, medium-infrared spectroscopy and Raman spectroscopy.
The analytical method of near-infrared (NIR) spectroscopy is a widespread technique, which is used both in the laboratory and in online operation. The combination of NIR spectroscopy with chemometric evaluation methods for special measurement tasks is likewise known per se from the prior art as described in, for example, DE 02139269, WO 97/41420, WO 98/29787, WO 99/31485, JP 11350368, WO 20002/0834, JP 2000146835, JP 2000298512, WO 2002/04394, WO 2002/12969, WO 95/31709, U.S. Pat. Nos. 5,707,870, 5,712,481, and WO 2000/68664.
Spectroscopic analysis techniques for determining the chemical properties of polymers and/or physical properties of polyurethane foams, both in the laboratory and in online operation, are known from “A review of process near infrared spectroscopy: 1980-1994” (J. Workman, J. Near Infrared Spectroscopy 1, 221-245 (1993)). The advantages of combining optical fibers and an NIR spectrometer, compared with using medium-infrared spectroscopy, are known from Khetty. See “In-line monitoring of polymeric processes” Antec '92, 2674-2676.
In order to use NIR spectroscopy in the field of quantitative determinations, the analytical method is frequently used in combination with chemometric evaluation methods. For example, it is customary to use the partial least-squares (PLS) method in this case, as can be found and described, for example, by Raphael Vieira in “In-line and In Situ Monitoring of Semi-Batch Emulsion Copolymerizations Using Near-Infrared Spectroscopy” J. Applied Polymer Science, Vol. 84, 2670-2682 (2002), or by T. Rohe in “Near Infrared (NIR) spectroscopy for in-line monitoring of polymer extrusion processes” Talanta 50 (1999) 283-290, or by C. Miller in “Chemometrics for on-line spectroscopy applications—theory and practice”, J. Chemometrics 2000; 14:513-528 and in“Multivariate Analysis of Near-Infrared Spectra Using G-Programming Language” J. Chem. Inf. Comput. Sci. 2000, 40, 1093-1100.
The use of NIR techniques for special measurement tasks is furthermore known and described in, for example, WO 00/02035 (Determination of organic acids in organic polymers), U.S. Pat. No. 5,717,209 (Spectral analysis of hydrocarbons), U.S. Pat. No. 6,228,650; WO 99/31485 (Monitoring the separation of chemical components in an alkylation process with acid catalyst), U.S. Pat. No. 6,339,222; WO 00/68664 (Determination of ionic species in pulp liquor), and DE 10005130 A1 (Monitoring of polymer processes, determination of NCO in PU).
A review of the use of multivariate chemometric calibration models in analytical chemistry is also provided by “Multivariate Calibration”, Jörg-Peter Conzen, 2001, ISBN 3-929431-13-0.
In the prior art, however, such spectroscopic methods are not used for isocyanate isomer mixtures.